The treatment of waste sludge containing a certain percentage of solids has in the past been approached in various ways. In some operations, separate units to thicken the sludge are not provided and, hence, what is used is a primary treatment unit to co-settle and thicken raw primary and waste secondary solids.
Many of the problems associated with the use of a primary clarifier as a thickener are the results of excessive solid retention time in the clarifier. Different designs of thickeners have been suggested in an attempt to overcome this problem and to efficiently accumulate suspension solids in effluents. The thickeners suggested so far have as a common feature a sloped conical bottom. However, systems allowing for quick settling of waste sludges are yet to be developed.
Attempts have also been made to treat sludges containing a certain amount of organic matter by creating a turbulence within the entire volume of the treatment apparatus to allow for constant mixing of the solids contained in the sludge. As a result, the supernatant liquid is treated but mixing of the entire sludges is required. This type of system does not avoid the necessity of accumulating non-degraded waste solids once digestion of the sludge has been performed. As the treatment apparatus in that case is usually not designed for accumulating and recuperating solids, the procedure can become lengthy and expensive.